Space Physics: Stars, the Solar System and the Big Bang

Our place in the universe

Space physics zooms out from the everyday to the largest scale there is. It begins close to home with the Solar System: the Sun and the planets, moons and other bodies that circle it, all held in their orbits by the pull of gravity. It then looks at the stars themselves, which are born from vast clouds of gas and dust, shine steadily for billions of years, and eventually die, some quietly and some in tremendous explosions that scatter new elements across space. Finally it steps back to the whole universe, which began about fourteen billion years ago in the event we call the Big Bang, and which is still expanding today. This module covers each of these: orbits and the Solar System, the life cycle of a star, how the universe began and the evidence for it, and the enormous distances involved, measured in light years.

Space physics words

Learn these terms before the star stages are matched up. They name the ideas used in this topic.

Match each star stage to what it is

A Sun-like star against a massive star

How a star ends its life depends almost entirely on how much mass it began with.

What keeps a satellite in orbit?

A satellite circles the Earth at a steady height. What provides the force that keeps it in its orbit?

• Gravity, the pull of the Earth
• Magnetism
• Friction with the air
• The wind

How far away the stars are

Space is so vast that ordinary units like kilometres become useless, so astronomers measure distance in light years. A light year is the distance that light, the fastest thing there is, travels in a whole year, and it is a staggering figure. Light from the Sun already takes about eight minutes to reach us, yet the Sun is our nearest star by far. The next nearest star is over four light years away, meaning we see it not as it is now but as it was more than four years ago. Look at a distant galaxy and you may be seeing light that set out millions of years ago. This is why telescopes are sometimes called time machines: the further out we look, the further back in time we see. Holding that scale in mind is the first step to thinking clearly about the universe.

Pick the true facts about space

Select every statement about space physics that is true.

• Gravity keeps planets in orbit around the Sun
• The universe began about fourteen billion years ago
• The universe is still expanding today
• The universe is fixed and never changes
• Stars live forever and never die

Order the life cycle of a Sun-like star

Put the stages in the life of a star like our Sun into the correct order.

• A cloud of gas and dust
• A protostar forms as it pulls together
• The long main sequence stage
• It swells into a red giant
• It leaves a white dwarf behind

Complete the space physics facts

The force that holds a planet in its orbit around the Sun is _____. The joining of small nuclei inside a star, releasing energy and building new elements, is called _____. The huge explosion that ends the life of a very massive star is a _____. The stretching of light from distant galaxies towards the red end, which is evidence that the universe is expanding, is called _____.

Tap the two pieces of Big Bang evidence

Read these four observations. Tap the TWO that are evidence for the Big Bang.

• the red shift of distant galaxies
• the cosmic microwave background radiation
• the changing phases of the Moon
• the red colour of Mars

Raisins in rising dough

To picture an expanding universe, imagine a lump of dough with raisins dotted through it, set to rise. As the dough swells, every raisin moves away from every other raisin, not because the raisins are travelling through the dough, but because the dough between them is growing. Sit on any raisin and look around, and all the others seem to be fleeing from you, with the most distant ones moving away fastest of all. That is exactly what we see when we look out at the galaxies: they are all rushing apart, and the far ones fastest. Space itself, like the dough, is expanding and carrying the galaxies with it. Run that expansion backwards in your mind and everything crowds together to a single hot, dense beginning, which is the picture at the heart of the Big Bang.

The space physics run

Answer each one correctly to keep your lives. Choose the best answer.

Reason about space

Read each situation and choose the best answer.

• You see light from a galaxy that is millions of light years away. What are you really seeing?
• The light from almost every distant galaxy is shifted towards the red. What does this tell us?
• A star far more massive than the Sun reaches the end of its life. What is likely left behind?

Build a space physics point

Choose the word for each gap to complete one point about space physics.

Explain space physics

A friend is revising space physics. Using what you have learned, explain the main ideas.

• Explain what holds the planets in orbit around the Sun
• Explain the life cycle of a star like the Sun
• Explain how a massive star ends differently
• Explain what the Big Bang is and one piece of evidence for it
• Explain why distances in space are measured in light years